Steam power plant having an improved control reserve

ABSTRACT

A method and to an apparatus for providing additional control power of a power plant process. The power plant process includes a steam turbine connected into a water vapor circuit, having at least one high-pressure part and a medium-pressure and/or no-pressure part, which are connected to one another via a cold intermediate superheating line, a steam generator and a condenser. A steam reservoir is provided, which is formed as a Ruths reservoir and in which an encapsulated PCM reservoir is integrated. To charge the steam reservoir, hot steam is taken from the cold intermediate superheating line, between the high-pressure and the medium-pressure and/or low-pressure part of the steam turbine, and for charging, and thus for providing additional control power, steam is taken from the steam reservoir and fed back into the water vapor circuit between the steam generator and the condenser.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the US National Stage of International Application No. PCT/EP2018/057178 filed 21 Mar. 2018, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2017 204 854.9 filed 22 Mar. 2017. All of the applications are incorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to a steam power plant having an improved control reserve.

BACKGROUND OF INVENTION

The proportion of renewable energies in the energy mix is constantly growing and is thus an essential building block in realizing the energy revolution. The aim is to increase the proportion of renewable energies. The challenges in the use of renewable energies lie in particular in the temporally varying provision characteristics.

FIG. 5 shows a diagram with two functions of power over time. The temporal deviation of the supply and demand of electricity is illustrated in the form of energy consumption 1 (load) and wind power production 2 as an example of renewable energies. Only in very few cases does the demanded amount of energy correspond to the provision and to the restricted control possibilities. Either stored energy 3 has to serve as the load 1 or surplus wind energy 4 from wind power production 2 is provided. Furthermore, renewable energies are not available in a uniformly regionally distributed manner. This has the consequence that increased network expansion is necessary if use cannot be made of any alternatives. This behavior leads to a control power market. The ability either to consume energy for a short time (negative control power) or to produce energy for a short time (positive control power) is rewarded.

In order to be able to participate in this market, it is necessary for the flexibility of the electricity production of steam power plants or combined gas and steam power plants to be increased.

In a steam power plant, positive control power in the range of seconds can be provided for example in that, for a short time, the turbine inlet valves are opened further and, at short notice, more power is generated through retrieval of steam from the boiler. The throttling of the main condensate in the condenser and additional firing in the steam generator are also known measures for providing more power at short notice.

An increase in the flexibility of a steam circuit in a steam power plant is furthermore also possible using heat accumulators. The functionality and efficiency of a heat accumulator is largely dependent on the location at which and the manner in which a heat accumulator is integrated in the steam circuit. The placement of suitable technology at a suitable location is always the aim.

There are no existing standard solutions on the market for the integration of a heat accumulator in a power plant. Different accumulator solutions are known from the prior art. Here, a distinction is made between sensible and latent heat accumulators.

As sensible heat accumulators, warm-water or pressurized-water accumulators or P2H solutions are known. In this case, use is made of heat from the steam circuit, or directly of electricity, for the purpose of producing warm water at the level of a district heating line. However, said solutions can be used only for negative control power.

Ruths accumulators are also known. FIG. 1 shows a Ruths accumulator in the form of a steam accumulator which consists substantially of a large pressure vessel 6. Since the heat at the end is stored as sensible heat in the water, such accumulators deliver only conditionally constant steam temperatures and, owing to economy and especially size, can be used only for a short time. Scaling is generally realized via the number of apparatuses.

Liquid-salt accumulators are also known. Liquid-salt accumulators are generally used in the steam circuit at high temperatures. The heat is stored as sensible heat in the liquid salt. Here, the performance is mainly dependent on the temperature difference of the heat exchanger and the pump power. Since, here, the energy is generally stored at a very high level, it is necessary for all the components to be very well insulated in order to avoid losses.

Batteries for storing electrical energy are also known. The electrical energy may be used for heating. However, the prices for battery solutions are relatively high.

As a latent heat accumulator, phase change materials (PCMs) are known. Macroencapsulated PCMs are already known from DE 10 2014 203 545 A1. DE 10 2014 203 545 A1 describes in particular the production method for the capsules.

SUMMARY OF INVENTION

It is the object of the invention to specify a method and a device by which additional control power can be provided in a power plant process.

The object of the invention directed at a method comprises a steam turbine which is connected into a steam circuit and which has at least one high-pressure and one medium- and/or low-pressure part, which are connected to one another via a cold intermediate superheating line (KZÜ), has a steam generator and has a condenser. Here, provision is made of a steam accumulator, which is a Ruths accumulator and into which an encapsulated PCM accumulator is integrated. For the purpose of charging, hot steam is extracted from the cold intermediate superheating line (KZÜ) between the high-pressure and the medium- and/or low-pressure part of the steam turbine. For the purpose of discharging, and thus for the purpose of providing additional control power, steam is extracted and is fed back into the steam circuit between the steam generator and the condenser.

The object of the invention directed at a power plant comprises a steam turbine which is connected into a steam circuit and which has at least one high-pressure and one medium- and/or low-pressure part, which are connected to one another via a cold intermediate superheating line (KZÜ). For the purpose of storing and providing additional control power, provision is made here of a steam accumulator, which is a Ruths accumulator and into which an encapsulated PCM accumulator is integrated. In this case, the steam accumulator, for the purpose of being charged with hot steam, is connected to the cold intermediate superheating line (KZÜ) between the high-pressure and the medium- and/or low-pressure part of the steam turbine, and, for the purpose of being discharged, is connected to the steam circuit between the steam generator and the condenser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Ruths accumulator in the form of a steam accumulator.

FIG. 2 shows a Ruths accumulator according to the invention.

FIG. 3 shows an alternative embodiment of the invention with a Ruths accumulator.

FIG. 4 shows an embodiment of a capsule in which a PCM is arranged.

FIG. 5 shows a diagram with two functions of power over time.

FIG. 6 shows the integration according to the invention of a Ruths accumulator with encapsulated PCM materials into a steam power plant.

DETAILED DESCRIPTION OF INVENTION

The invention provides a novel concept for integrating a macroencapsulated PCM (phase change material) accumulator into a steam circuit. FIG. 2 shows a Ruths accumulator 5 according to the invention with an integrated macroencapsulated PCM. FIG. 3 shows an alternative embodiment of the invention with a Ruths accumulator 5 with an externally connected tube register system 8 with an externally arranged PCM. The Ruths accumulators in FIGS. 2 and 3 use the saturated water of the Ruths accumulator 5 as a heat carrier fluid. Each variant can accommodate only a particular saturated water volume. Taking the water volume of a pure Ruths accumulator 5 as a reference, the Ruths accumulator with macroencapsulated PCM 7 displaces close to 50% of the water content.

The present invention is based firstly on the consideration of integrating these encapsulated PCM materials 7 into a Ruths accumulator 5. The steam accumulator obtained in this way has a significantly greater capacity per volume and is thus significantly superior to a Ruths accumulator from the prior art. The present invention is based secondly on the consideration of integrating the modified Ruths accumulator according to the invention into a steam power plant.

Furthermore, the accumulator according to the invention is able to deliver steam parameters which are constant over a relatively long time, since the heat is not stored as sensible heat. During the discharge of the accumulator or solidification of the PCM material, heat is released continuously at a constant temperature level. Consequently, it is possible to use said accumulator even at locations where the temperature differences are relatively small.

The invention is described in more detail below on the basis of FIG. 6:

FIG. 6 shows the integration according to the invention of a Ruths accumulator 5 with encapsulated PCM materials 7 into a steam power plant 10. A steam turbine 11 having a high-pressure part 12, a medium-pressure part 13 and a low-pressure part 14 on a common shaft 15 can be seen. The steam that has been expanded in the high-pressure part 12 of the steam turbine 11 flows via a cold intermediate superheating line KZÜ 17 back into the steam generator, is heated there, and is fed to the medium-pressure part 13 of the steam turbine 11. The heat accumulator according to the invention is connected to the cold intermediate superheating line 17.

For the purpose of charging the Ruths accumulator 5, steam is extracted from the cold intermediate superheating means (KZÜ) 17 of the steam turbine. The cold intermediate superheating means 17 constitutes a reliable supply rail in the steam power plant since the steam can be extracted even before the start-up of the turbine and also during turbine operation. The quantity extracted from the KZÜ 17 for charging the Ruths accumulator 5 is limited by the minimum throughflow in the intermediate superheater boiler and the minimum throughflow in the medium-pressure (MD) and low-pressure (ND) turbine.

The extracted steam condenses in the Ruths accumulator 5, releases its condensation heat and, in this way, melts the PCM 7. The PCM phase change temperature is selected as follows:

T _(PCM) <=T _(sat)(p _(MIN KZÜ))−15 K.

The 15 K (Kelvin) constitute the temperature difference between the temperature of the water/steam stock of the Ruths accumulator 5 and the temperature of the integrated PCM material 7. The control valve 18 is advantageous for setting the steam pressure according to the accumulator charging temperature. Since steam can be extracted quickly according to the operating time of the control valve 18, the power of the power plant is quickly reduced and thus participation on the negative control power market is possible even for low block powers.

In the case of steam extraction prior to the start-up of the turbine, the storage of energy is already realized during the turbine bypass operation. During discharge, steam is extracted in an uncontrolled manner from the Ruths accumulator 5. In this way, the PCM 7 solidifies and releases its heat to the water, which evaporates. In this way, steam extraction at constant temperature is possible. The steam generated in the Ruths accumulator 5 with integrated PCM 7 has a temperature:

T _(steam) <=T _(PCM)−15 K.

The 15 K (Kelvin) constitute the temperature difference between the temperature of the integrated PCM material and the temperature of the water/steam stock of the Ruths accumulator 5.

The steam is conducted to the low-pressure preheater 19 and used for transfer of heat to the main condensate. In this way, the extraction of steam from the steam turbine 11 is minimized or even prevented. Consequently, more steam remains in the turbine 11 and greater power is generated, which can be used as control power.

Additionally, the higher saturated steam temperature of the stored steam leads to the increase in the temperature of the main condensate after the low-pressure preheater 19, which is supplied by means of stored steam. This additionally brings about a reduction in the steam requirement of the feed-water vessel which follows, this in turn meaning that more steam remains in the steam turbine 11 and, additionally, more turbine power is generated.

Participation on the positive control power market is thus possible. The advantage of this integration is that, even at relatively low pressures, steam can be extracted from the KZÜ 17 and stored as heat in the Ruths accumulator 5 with integrated PCM 7, with a large influence on the production of electricity still however being possible, since the accumulator discharge is realized at high block powers. In this way, a cost-effective concept with relatively little heat loss is possible.

The following table lists a selection of salt hydrates which satisfy the requirements for the encapsulated phase change material (PCM) 7 for the accumulator applications covered. These include for example the PCM 95% NaNO₃+5% NaCl, which consists of a mass ratio of 95% of sodium nitrate to 5% of sodium chloride. For said materials, the material properties, such as specific heat capacity, heat conductivity, density and also the entropy of fusion and melting temperature, are also known.

PCM 95% NaNo3 + NaNo3 − 46% NaNo3 + parameter 5% NaCl NaOH 54% KNO3 h_(melt) kJ/kg 212 270 100 T_(melt) ° C 282 258 222 C_(pPCM) kJ/(kg * K) 1.78 1.15 142 λ W/(kg * K) 0.59 0.75 0.45 ρ kg/m³ 2260 2195 2100

From the table, a comparison of the enthalpies of fusion and of the specific heat capacities reveals that although NaNo₃—NaOH has a poorer heat capacity value C_(pPCM), it releases significantly greater enthalpy of fusion h_(melt) for the phase change. Moreover, NaNo₃—NaOH has a higher heat conductivity and is thus able to transport the energy stored in the melted PCM to the heat transfer surface of the capsule more effectively. NaNo₃—NaOH is therefore the PCM advantageous for the invention.

FIG. 4 shows an embodiment of a capsule 20 in which a PCM 21 is arranged. A high-grade steel tube with a diameter of between 20 and 100 mm, advantageously 50 mm, a length of between 200 mm and 1000 mm, advantageously 400 mm, and a wall thickness of between 0.2 and 5 mm, advantageously 1 mm, is used as a capsule 20. After the capsule 20 has been filled with PCM 21, pressing-together and welding is realized at both ends. In this way, at both capsules 20, a 5 mm portion is formed in each case. Moreover, the capsule 20 is filled with PCM only to between 50 and 95%, advantageously 80%, since the PCM volume is changed during the phase change.

The invention allows a cost-effective integration of an accumulator into a steam circuit, increases the flexibility of the steam circuit and thus allows the participation on the negative and positive control power market.

Here, it is particularly advantageous that steam can be stored even starting at the minimum load of the power plant and also even during the start-up of the turbine.

The retrieval of steam is realized here at the high power level of the power plant.

It is also advantageous here that the integration is managed in this case with minimal additional outlay, the latter mainly being limited to the Ruths accumulator 5 to be integrated with integrated PCM 7 and connecting tube lines. The integration requires no structural changes in the low-pressure feed-water preheating means. A control valve 18 is provided in the means for steam extraction 11 from the KZÜ 17 in order to set the saturated steam parameters according to the selected PCM. The steam retrieval may also be realized in an uncontrolled manner.

It is particularly advantageous that the accumulator generates saturated steam whose heat can be transferred directly from the low-pressure preheater 19 to the main condensate. In addition, the existing delivery of the secondary condensate into the main condensate stream by the secondary condensate pump allows complete utilization of the retrieved energy.

Since the Ruths accumulator 5 with integrated PCM 7 produces saturated steam at a higher pressure level than the turbine bleeding means associated with the low-pressure preheater 19, the temperature of the main condensate is increased after the low-pressure preheater, which is supplied by means of stored steam, this advantageously reducing the steam requirement of the preheaters which follow. This has the effect that, not only is turbine extraction steam replaced by steam from the accumulator at the location where the accumulator is integrated, but also turbine extraction steam is reduced at the low-pressure preheater which follows (the feed-water vessel in the present case).

A possibly additional accumulator for providing supporting steam for the feed-water vessel is avoided by this concept, or the requirements thereof are reduced. 

1. A method for providing additional control power of a power plant process comprising a steam turbine which is connected into a steam circuit and which has at least one high-pressure and one medium- and/or low-pressure part, which are connected to one another via a cold intermediate superheating line, has a steam generator and has a condenser, the method comprising: providing a steam accumulator, wherein the steam accumulator is a Ruths accumulator into which an encapsulated PCM accumulator is integrated, for the purpose of charging, extracting hot steam from the cold intermediate superheating line between the high-pressure and the medium- and/or low-pressure part of the steam turbine, and for the purpose of discharging, and thus for the purpose of providing additional control power, extracting steam which is fed back into the steam circuit between the steam generator and the condenser.
 2. A power plant, comprising: a steam turbine which has a steam generator and a condenser and which is connected into a steam circuit and which has at least one high-pressure and one medium- and/or low-pressure part, which are connected to one another via a cold intermediate superheating line, a steam accumulator for the purpose of storing and providing additional control power, wherein the steam accumulator is a Ruths accumulator into which an encapsulated PCM accumulator is integrated, wherein the steam accumulator, for the purpose of being charged with hot steam, is connected to the cold intermediate superheating line between the high-pressure and the medium- and/or low-pressure part of the steam turbine, and, for the purpose of being discharged, is connected to the steam circuit between the steam generator and the condenser. 